A stepping motor is driven steppingly in response to input pulses and many types of stepping motors such as two phase or multiphase types are well known. For instance, a five-phase stepping motor however has the following drawbacks. Namely, a single pulse may drive the motor at angles of 0.72.degree. or 0.36.degree., but such rotation angles are still rough and does not bring about a smooth rotation of the motor. Further, there is found mechanical sympathetic vibration between the motor and the drive frequency in which it happens that the motor cannot be driven.
Then, it is required for such a drive system that electric currents of motor coils A and B and so forth are controlled and a direction of a composite torque vector is gradually varied so as to drive the motor steppingly at angles of 0.072.degree. or 0.036.degree. with a single pulse by dividing the angles of 0.72.degree. into one tenth or one twentieth in order to overcome the above-mentioned particular drawbacks of the stepping motor.
To satisfy such a requirement, the following method or means is adopted conventionally. Namely, as shown in FIG. 14, four output elements Tr1, Tr2, Tr3 and so forth are connected to bridge every one of the coils of the motor. A +V voltage is impressed to each of the motor coils A, B and so forth so that the drive current may flow through the motor coils A, B and so forth. The drive current through the motor coils A and B, is detected by means of sensing resistances R1 and so forth for detecting the drive current so as to take out the result one by one. An output element Tr is made independent for every one of the motor coils A, B and so forth so as to carry out a switching control so that microangle drive of the motor may be carried out by gradually varying the torque vectors from E to O to E. Accordingly, in the case of the five-phase stepping motor, five motor coil control circuits M1-M5 control the drive current for the five motor coils A, B, C, D, and E. Each of output elements Tr1-Tr20 are controlled by means of suitable switching so as to control the current for the stepping motor.
However, this system has the following disadvantages or drawbacks.
(1) Since the four output elements Tr1, Tr2, Tr3, Tr4 and so forth are connected into a bridge for every phase so as to control the motor by detecting the drive current, in the case of a stepping motor of n-phases, a motor coil control circuit M has to be prepared for n-pieces; namely, 4n-pieces of an output element Tr (20 pieces for a 5-phase pulse motor), n-pieces of a detecting circuit (5 pieces for a 5-phase pulse motor). Therefore, the entire circuit becomes complex and therefore increases manufacturing cost.
(2) Since a +V voltage is controlled every phase, switching control is necessary for at least n-pieces of the output element Tr; namely switching control of a rated current is carried out by the output element of (n-1) and switching control of a microangle drive current is controlled by the one remaining output element Tr. As a result, it brings about disadvantages of electric power loss due to generation of heat of the n-piece output elements Tr and produces switching noises.
(3) Further, since switching control is given to a +V voltage every phase, a ripple occurs to a rated current and a microangle drive current flows through the motor coils A, B and so forth. As a result, it lacks stability of a stop position when the motor stops.